The House Judiciary Committee filed a lawsuit Wednesday to enforce its subpoena for former White House counsel Don McGahn, launching a separation-of-powers battle in the courts that might not be quickly resolved.

The lawsuit focuses on McGahn’s key role in the Robert S. Mueller III-led special counsel probe when it comes to potential misconduct by President Donald Trump. It points out that the Mueller report section that focuses on whether Trump tried to obstruct the investigation mentions his statements more than 160 times.

And it argues that McGahn’s testimony is crucial to the House’s “most solemn constitutional responsibility” to determine whether to recommend articles of impeachment.

“McGahn can give a firsthand account of his discussions with President Trump and other White House aides about the president’s actions and their reactions to them,” the lawsuit states. “In addition, McGahn was responsible for facilitating communications between the White House and DOJ, and advising the president on the propriety of such communications.”

The lawsuit asks the courts to issue an injunction ordering McGahn to appear and testify before the committee about the Mueller report. It is the first lawsuit House Democrats have filed to force an administration official to testify since Trump announced he was taking a fight-all-the-subpoenas strategy.

The Judiciary Committee took the issue to the courts almost two months after the House passed a resolution that authorized the legal action. The measure passed 229-191 on a pure party-line vote.

And it comes more than two months after Trump ordered McGahn not to testify at a House Judiciary hearing set for May 21, saying the former counsel is not legally required to comply with a congressional subpoena.

In a May 20 letter to House Judiciary Chairman Jerrold Nadler, current White House counsel Pat Cipollone wrote that an internal Justice Department opinion determined Congress cannot compel the president’s senior advisers to testify about their official duties.

The Judiciary Committee lawsuit contends that the president has waived executive privilege over much of the testimony it seeks as part of “an urgent oversight duty to protect ongoing investigations from improper interference.” It says the committee tried to make accommodations for McGahn.

The lawsuit also contends that McGahn would have “significant evidence of the president’s motivations for his actions.”

“McGahn’s firsthand account of the specific words, tone, emotional state, body language and other actions of the president when he instructed McGahn to have special counsel Mueller fired — and then when the president ordered McGahn to create a document falsely contradicting a press account of the incident — would be of critical aid” to the Judiciary Committee, the lawsuit states.